Despite the growing focus on digital communication technologies, the 4-20 mA analog output remains one of the most dominant types of analog output in the industry today. Another popular type of analog output is the 0-10 VDC output.
There exist different types of 4-20 mA loops, where the two-wire loop version is by far the most widespread. Three and four wire 4-20 mA variants are similar in their fundamental working principle and thus we will not treat them here.
The basic concept behind the two-wire 4-20 mA current loop is to power the device through the same two-wires that are part of the analog measurement circuit.
An example of a two-wire current loop is shown in the figure below.
The main current loop comprises a DC power supply, a 250 Ω resistor and a transmitter connected in series.
A voltmeter is connected in parallel with the resistor to measure the voltage across it.
The transmitter regulates the amount of current that flows through the circuit as a function the process variable measured by the transmitter, e.g. temperature or pressure. A typical linear analog output scaling for a 1 bar full-scale pressure transmitter is a loop current of 4 mA at 0 bar and 20 mA at 1 bar.
Since the amount of current is the same throughout the entire main loop at any given point in time, the voltage across the 250 Ω resistor becomes a direct measure of the process variable. For example, if the pressure is 0 bar, the transmitter regulates the loop current to 4 mA and consequently the voltage measured across the resistor is 1 V (due to Ohm’s law – link here). Similarly, if the pressure is 1 bar the transmitter regulates the loop current to 20 mA which corresponds to 5 V across the resistor.
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